That is what many people were asking Wednesday, after news broke that a health care worker had flown commercially at a time she was supposedly monitoring herself for a possible Ebola infection.

Amber Vinson, 29, is the third person to be diagnosed with Ebola in the United States -- after Thomas Eric Duncan, the Liberian national who fell ill after coming to Texas, and Nina Pham, who like Vinson had cared for Duncan at Dallas' Texas Health Presbyterian Hospital.

As with Pham, many questions surround how Vinson contracted Ebola despite efforts to prevent health care workers like them from getting the disease. Now they are compounded with others about safeguards for those who had contact with Duncan, who succumbed to the disease on October 8, to prevent the disease from spreading further.

One of authorities' biggest fears is that Ebola could be transmitted to the general public in a confined, crowded place. Someplace like, perhaps, a busy airport or an airplane -- like that of Frontier Flight 1143, which Vinson flew Monday with a slightly elevated temperature from Cleveland to Dallas.

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Photos:The Ebola epidemic

Photos:The Ebola epidemic

An Ebola survivor participates in a study in Monrovia, Liberia, on June 17. The country launched a five-year study to unravel the mystery of the long-term health effects that plague survivors of the viral disease. Since the epidemic started more than a year ago in a remote village in Guinea, more than 11,000 people have died, the vast majority in three West African nations, according to the latest numbers from the World Health Organization. And that number is believed to be low, since there was widespread under-reporting of cases, according to WHO.

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Women in Monrovia celebrate after the World Health Organization declared Liberia Ebola-free on May 9. Other cases have recurred since, however. Two people in Liberia have died of the disease since the end of June, just weeks after the WHO declared the nation free of the disease.

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A man walks past an Ebola awareness painting in Monrovia on March 22.

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Soldiers from the U.S. Army's 101st Airborne Division walk across the tarmac at Campbell Army Airfield before reuniting with their families at a homecoming ceremony March 21 in Fort Campbell, Kentucky. The 162 soldiers were deployed in Liberia, where they helped fight the spread of Ebola.

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Relatives weep for a loved one who it was believed died from Ebola, at a graveyard on the outskirts of Monrovia on March 11.

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Doctors Without Borders staffer Alex Eilert Paulsen watches as mattresses and bed frames burn at the Ebola Treatment Unit in Paynesville, Liberia, on January 31. The organization reduced its number of beds from 250 to 30 as gains were made in battling the virus.

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Pauline Cafferkey, a Scottish woman diagnosed with Ebola, is put on a plane in Glasgow, Scotland, on December 30, 2014. Cafferkey, a 39-year-old nurse who volunteered in Sierra Leone, was being transported to London for treatment.

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A child who survived the Ebola virus is fed by another survivor at a treatment center on the outskirts of Freetown, Sierra Leone, on November 11, 2014.

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Health workers in Monrovia cover the body of a man suspected of dying from the Ebola virus on October 31, 2014.

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Photos:The Ebola epidemic

Kaci Hickox leaves her home in Fort Kent, Maine, to take a bike ride with her boyfriend on October 30, 2014. Hickox, a nurse, recently returned to the United States from West Africa, where she treated Ebola victims. State authorities wanted her to avoid public places for 21 days -- the virus' incubation period. But Hickox, who twice tested negative for Ebola, said she would defy efforts to keep her quarantined at home.

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Health officials in Nairobi, Kenya, prepare to screen passengers arriving at the Jomo Kenyatta International Airport on October 28, 2014.

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U.S. President Barack Obama hugs Ebola survivor Nina Pham in the Oval Office of the White House on October 24, 2014. Pham, one of two Dallas nurses diagnosed with the virus, was declared Ebola-free after being treated at a hospital in Bethesda, Maryland. The other nurse, Amber Vinson (not pictured), was treated in Atlanta and also declared Ebola-free.

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Health workers in Port Loko, Sierra Leone, transport the body of a person who is suspected to have died of Ebola on October 21, 2014.

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Health workers bury a body on the outskirts of Monrovia on October 20, 2014.

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Photos:The Ebola epidemic

Garteh Korkoryah, center, is comforted during a memorial service for her son, Thomas Eric Duncan, on October 18, 2014, in Salisbury, North Carolina. Duncan, a 42-year-old Liberian citizen, died October 8 in a Dallas hospital. He was in the country to visit his son and his son's mother, and he was the first person in the United States to be diagnosed with Ebola.

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Boys run from blowing dust as a U.S. military aircraft leaves the construction site of an Ebola treatment center in Tubmanburg, Liberia, on October 15, 2014.

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Aid workers from the Liberian Medical Renaissance League stage an Ebola awareness event October 15, 2014, in Monrovia. The group performs street dramas throughout Monrovia to educate the public on Ebola symptoms and how to handle people who are infected with the virus.

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Ebola survivors prepare to leave a Doctors Without Borders treatment center after recovering from the virus in Paynesville, Liberia, on October 12, 2014.

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A man dressed in protective clothing treats the front porch of a Dallas apartment on October 12, 2014. The apartment is home to one of the two nurses who were diagnosed with Ebola after treating Thomas Eric Duncan, a Liberian national who traveled to Dallas and later died from the virus.

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A woman crawls toward the body of her sister as a burial team takes her away for cremation October 10, 2014, in Monrovia. The sister had died from Ebola earlier in the morning while trying to walk to a treatment center, according to her relatives.

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A man digs a grave on October 7, 2014, outside an Ebola treatment center near Gbarnga, Liberia.

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A person peeks out from the Dallas apartment where Thomas Eric Duncan, the first person diagnosed with the Ebola virus in the United States, was staying on October 3, 2014.

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A girl cries as community activists approach her outside her Monrovia home on October 2, 2014, a day after her mother was taken to an Ebola ward.

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A health official uses a thermometer September 29, 2014, to screen a Ukrainian crew member on the deck of a cargo ship at the Apapa port in Lagos, Nigeria.

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Workers move a building into place as part of a new Ebola treatment center in Monrovia on September 28, 2014.

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Medics load an Ebola patient onto a plane at Sierra Leone's Freetown-Lungi International Airport on September 22, 2014.

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A few people are seen in Freetown during a three-day nationwide lockdown on September 21, 2014. In an attempt to curb the spread of the Ebola virus, people in Sierra Leone were told to stay in their homes.

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Supplies wait to be loaded onto an aircraft at New York's John F. Kennedy International Airport on September 20, 2014. It was the largest single shipment of aid to the Ebola zone to date, and it was coordinated by the Clinton Global Initiative and other U.S. aid organizations.

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A child stops on a Monrovia street September 12, 2014, to look at a man who is suspected of suffering from Ebola.

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After an Ebola case was confirmed in Senegal, people load cars with household items as they prepare to cross into Guinea from the border town of Diaobe, Senegal, on September 3, 2014.

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A health worker wearing a protective suit conducts an Ebola prevention drill at the port in Monrovia on August 29, 2014.

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A burial team from the Liberian Ministry of Health unloads bodies of Ebola victims onto a funeral pyre at a crematorium in Marshall, Liberia, on August 22, 2014.

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Dr. Kent Brantly leaves Emory University Hospital on August 21, 2014, after being declared no longer infectious from the Ebola virus. Brantly was one of two American missionaries brought to Emory for treatment of the deadly virus.

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An Ebola Task Force soldier beats a local resident while enforcing a quarantine on the West Point slum on August 20, 2014.

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Local residents gather around a very sick Saah Exco, 10, in a back alley of the West Point slum on August 19, 2014. The boy was one of the patients that was pulled out of a holding center for suspected Ebola patients after the facility was overrun and closed by a mob on August 16. A local clinic then refused to treat Saah, according to residents, because of the danger of infection. Although he was never tested for Ebola, Saah's mother and brother died in the holding center.

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Workers prepare the new Ebola treatment center on August 17, 2014.

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Liberian police depart after firing shots in the air while trying to protect an Ebola burial team in the West Point slum of Monrovia on August 16, 2014. A crowd of several hundred local residents reportedly drove away the burial team and their police escort. The mob then forced open an Ebola isolation ward and took patients out, saying the Ebola epidemic is a hoax.

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A health worker disinfects a corpse after a man died in a classroom being used as an Ebola isolation ward August 15, 2014, in Monrovia.

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Aid worker Nancy Writebol, wearing a protective suit, gets wheeled on a gurney into Emory University Hospital in Atlanta on August 5, 2014. A medical plane flew Writebol from Liberia to the United States after she and her colleague Dr. Kent Brantly were infected with the Ebola virus in the West African country.

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Members of Doctors Without Borders adjust tents in the isolation area in Kailahun on July 20, 2014.

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Boots dry in the Ebola treatment center in Kailahun on July 20, 2014.

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Dr. Jose Rovira of the World Health Organization takes a swab from a suspected Ebola victim in Pendembu, Sierra Leone, on July 18, 2014.

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Red Cross volunteers disinfect each other with chlorine after removing the body of an Ebola victim from a house in Pendembu on July 18, 2014.

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A scientist separates blood cells from plasma cells to isolate any Ebola RNA and test for the virus April 3, 2014, at the European Mobile Laboratory in Gueckedou, Guinea.

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Health specialists work March 31, 2014, at an isolation ward for patients at the facility in southern Guinea.

No, according to the head of the Centers for Disease Control and Prevention.

Dr. Thomas Frieden said -- given what happened with Duncan and a nurse who got Ebola from him -- the health care care worker should not have traveled on a commercial airline.

"The CDC guidance in this setting outlines the need for what is called controlled movement," Frieden said. "That can include a charter plane, a car, but it does not include public transport."

Is it possible she didn't know that?

Yes.

This is a big question, because it's harder to fault Vinson if no one instructed her not to fly commercially.

Another infectious disease expert -- Dr. Art Reingold, head of epidemiology at the University of California-Berkeley School of Public Health -- says he hadn't heard "any firm recommendations" on traveling for those, like Vinson, who were supposed to be monitoring themselves.

"If I had travel plans, I would probably put them on hold," Reingold said. "But I haven't heard anyone say that should be a policy."

Vinson called the CDC on Monday to report she had a temperature of 99.5 Fahrenheit and inform the agency she was about to board a plane back to Dallas, the official said.

No one at the CDC told her she couldn't or shouldn't fly, according to the official.

Was Vinson contagious when she flew?

Not necessarily.

A person can't pass on Ebola the moment they contract it. Rather, someone is only contagious when they are showing symptoms -- which, according to the CDC, include a severe headache, muscle pain, diarrhea, vomiting, abdominal pain and a fever greater than 101.5 degrees Fahrenheit (38.6 degrees Celsius). And the National Institutes of Health notes that a temperature over 100.4 degrees usually means a person has a fever caused by an infection or illness.

That latter measure -- whether a person's temperature is 100.4 degrees or higher -- is the first item on a CDC checklist for those being evaluated for Ebola. The CDC is considering lowering that threshold for the virus, according to an official familiar with the situation.

What's not known is how that varies from her normal body temperature (a baseline that can differ depending on the person, time of day or other factors), how it might have spiked during the flight or how she was feeling beyond her temperature.

And the fact of the matter is Vinson wasn't just any traveler. She, like fellow Ebola sufferer and Dallas health care worker Nina Pham, had "extensive contact" with Duncan between September 28 and 30 when he had "extensive production of body fluids" such as vomit and diarrhea, Frieden said.

In other words, Vinson had special reason to be careful about her interactions with others.

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Why aren't all travelers temperatures checked in Cleveland?

Because that's not the U.S. policy -- at least right now.

Passengers' temperatures are checked at five American airports: New York's Kennedy, Newark's Liberty, Atlanta's Hartsfield-Jackson, Chicago's O'Hare and Washington's Dulles. Cleveland's Hopkins International Airport is not on the list.

Even then, those temperature checks are only for those who have come from West Africa to the United States. Even if Vinson had flown from, say, O'Hare to Dallas, she wouldn't have been checked because she hadn't recently flown internationally.

Could this policy change? Sure. But Dr. Joseph McCormick points out that testing temperatures wouldn't catch those, like Duncan, who got exposed days before and didn't show any symptoms while traveling. It would end up getting many more people who have fevers for other reasons, especially once flu season kicks into high gear.

"I think that's more than enough," McCormick, dean of the University of Texas School of Public Health's Brownsville campus, said of the current checks on those coming from West Africa. "I'm not persuaded (testing all travelers' temperature) is going to be really doing anything."

There were 132 people who flew, with Vinson, on a Frontier flight from Cleveland to Dallas earlier this week. And each one of them will be interviewed by authorities and could be monitored. But that doesn't mean they are likely to show symptoms of Ebola.

The virus doesn't spread, through the air, like the flu or cold. It is transmitted through bodily fluids such as urine, saliva, sweat or feces, none of which you'll likely exchange with every fellow passenger on your flight.

Still, a lot depends on where you were sitting and what symptoms Vinson had during the flight. If you rubbed elbows with her, mistakenly shared a drink or something else, that might be cause for heightened for concern.

"If I had been on that plane, I personally would not have been worried about getting Ebola," Reingold said. "Now, if that person vomited on my lap, I would be saying something quite different."

Wasn't Vinson under quarantine?

Photos:Who's who in the Ebola outbreak?

Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Thomas Eric Duncan is a Liberian resident who flew to Dallas to visit family and friends. He was the first Ebola patient diagnosed in the United States. He passed away on October 8.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Nina Pham, 26, is a Dallas nurse involved in Duncan's care who was diagnosed with Ebola, marking the first known transmission of Ebola in the United States. She had on a gown, gloves, mask and a shield during her multiple visits with Duncan, but there was a breach in protocol, health officials said.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Amber Vinson, 29, was the second nurse to be diagnosed with Ebola after treating Duncan at Texas Health Presbyterian Hospital.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – A nurse's assistant identified as Teresa Romero Ramos tested positive for Ebola after treating a Spanish missionary with the deadly virus in Madrid, Spain. Her case is the first recorded transmission of Ebola outside of West Africa during this outbreak.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Centers for Disease Control and Prevention Director Dr. Tom Frieden has led the effort to evacuate and treat American patients and has helped U.S. hospitals prepare for a possible outbreak at home. The CDC also has teams working in West Africa assisting with contact tracing and infection control.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Dr. Kent Brantly contracted Ebola while working as the medical director for Samaritan's Purse Ebola Care Center in Monrovia, Liberia. He was the first person to be treated with the experimental drug ZMapp and was the first patient to be brought home to the United States.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Nancy Writebol is an American missionary with SIM USA who also contracted Ebola in Liberia. She, too, was given ZMapp and flown back to the United States for treatment.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Dr. Margaret Chan has been the World Health Organization's director-general since 2006. Originally from China, she has a strong background in communicable diseases and infection control.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Texas Gov. Rick Perry has overseen the state's response to Duncan's case. State and local health officials are working with the CDC to monitor around 50 individuals who had contact with the Ebola patient while he was contagious.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – American Dr. Rick Sacra was delivering babies in a hospital in Liberia when he contracted Ebola. He was the first Ebola patient to be treated in The Nebraska Medical Center's biocontainment unit.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – American Ashoka Mukpo is a freelance cameraman who was working for NBC News in Liberia when he became ill with Ebola symptoms. He was flown to The Nebraska Medical Center on October 6.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Liberian President Ellen Johnson Sirleaf has been very outspoken about the international community's response to the Ebola outbreak in West Africa. Liberia has had the most cases and deaths of all the countries affected by the outbreak.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Alpha Conde is the president of Guinea, which has had more than 1,100 cases, including 739 deaths.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Ernest Bai Koroma is the president of Sierra Leone, which has had more than 2,400 cases, including 623 deaths.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – The well-known Dr. Sheik Humarr Khan, left, died after contracting Ebola while helping patients in Sierra Leone.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Joanne Liu is the international president of Medecins Sans Frontieres, also known as Doctors Without Borders. MSF has been on the ground in West Africa since the outbreak started and has played a key role in treating thousands of patients in Guinea, Liberia and Sierra Leone.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Spanish priest Manuel Garcia Viejo was diagnosed with Ebola while working in Sierra Leone. He was flown back to Spain for treatment before he died. A nurse's assistant who treated him in Spain is believed to have contracted the virus as well.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Patrick Sawyer collapsed after getting off a plane in Lagos, Nigeria, and later died. Health officials believe he was the start of the small outbreak in that country.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Dr. Gorbee Logan is one of two doctors for more than 85,000 people in Bomi County, Liberia. Logan says he has successfully treated Ebola patients with anti-retroviral drugs, which are commonly used to treat HIV.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Fatu Kekula has cared for four family members who had Ebola, keeping three alive without infecting herself using trash bags, rubber boots and a mask.

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Photos:Who's who in the Ebola outbreak?

Who's who in the Ebola outbreak? – Maj. Gen. Darryl Williams is commander of the U.S. military's Operation United Assistance in West Africa. The U.S. will be sending around 3,600 troops to the region to help fight the Ebola outbreak.

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Who's who in the Ebola outbreak? – Valerie Amos, the United Nations under-secretary-general for humanitarian affairs and emergency relief coordinator, has been leading the U.N.'s response to the outbreak.

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It depends what you mean by quarantine.

Vinson didn't face the same restrictions as four people who spent days in the same house as Duncan before his admission to Texas Health Presbyterian. They faced prosecution if they ventured into the general public before their quarantine was over.

But Vinson was being "monitored" for possible Ebola, like about 75 fellow health care workers who also were involved in Duncan's care, said Texas Health Resources' chief clinical officer Dr. Daniel Varga. So, too, were 48 other people in the community who'd had contact with Duncan, according to Clay Jenkins, the head of Dallas County's Homeland Security and Emergency Management office.

So what would such "monitoring" entail? Authorities have said those subject to it should monitor their temperatures daily, for signs of a fever, and have check-ins from health officials.

Did that happen as it should, in Vinson's case? Dallas Mayor Mike Rawlings told CNN that he doesn't think so.

"If she was being monitored correctly," he said, "I think she should have never gotten on that flight."

So who could have prevented this?

This air travel issue would never have happened had Duncan not contracted Ebola in Liberia, then came down with symptoms from the virus in Texas. And we wouldn't be talking about Vinson's travels had she not gone to Cleveland, or tried to come back to Dallas.

But there seems to be one thing that everybody agrees on: Health authorities made mistakes themselves.

Yes, the CDC is the top federal agency when it comes to infectious diseases. But as McCormick points out, it doesn't have any legal authority over state or local health agencies. So any policies it suggest can be moot, unless embraced at a more local level.

And obviously, such policies -- like, perhaps, that those being asked to "self-monitor" for Ebola not fly commercially -- are more futile if they're not communicated at all.

McCormick said "the buck stops" with the hospital, which has the primary responsibility to train and prepare its staff. He finds Texas Health Presbyterian Hospital officials' response to the latest case -- and the fact a CDC official wasn't there alongside them -- to be "disappointing."

"They were really pretty much trying to paper over the missteps," McCormick said. "... The evidence is just overwhelming is that this is not good."

The CDC has established Ebola response teams to send to hospitals within hours of a new case and promised to give hospitals a clear understand of how best to protect their workers. Clear communication is a big component in preventing the disease's spread, much like proper equipment.

One option being considered is to add those being monitored for Ebola -- people such as Vinson -- to a "no-board" list for air travelers the CDC operates with the Department of Homeland Security, according to a CDC official.

Sylvia Burwell, the U.S. Secretary of Health and Human Services, says government policy will continue to evolve every time there's a new case, a new mistake or a new breakthrough.

"Wherever possible we're learning from our experiences and doubling on our efforts to continue to improve processes and procedures," Burwell said Wednesday. "We're continuing to communicate and supporting implementation efforts on the ground."

That's important because, as the people of West Africa know all too well and the U.S. cases show, the margin for error is slim when it comes to Ebola.

As Dr. Frank Esper, an infectious disease specialist in Cleveland, says: "Ebola is a disease that is not very forgiving of mistakes."